The detectors used for the detection of Gamma and x-ray radiation, e.g. in CT, Dual-Energy CT, SPECT and PET systems, include direct-conversion radiation detectors, based on semiconductor direct-converter materials, such as CdTe, CdZnTe, CdZnTeSe, CdTeSe, CdMnTe, InP, TIBr2, HgI2. With these materials however, especially with a high radiation flux density necessary for CT devices, the polarization effect occurs.
Polarization refers to the reduction of the detected counting rate at high photon or radiation fluxes. The polarization is caused by the very low mobility of the charge carriers, above all of the electron holes, and through the concentration of intrinsic impurities in the semiconductor. The polarization thus arises through the reduction of the electric field as a result of fixed-location charges bound to the fault points which act as capture and recombination centers for the charge carriers created by the x-ray radiation. This results in a reduction of the charge carrier lifetime and mobility, which in its turn leads to a reduction of the detected count rate at the high radiation flux densities.
The polarization of the semiconducting direct converter material changes during a measurement process. This change of the electric field in its turn results in a change of the measured pulse heights and thus also has an effect on the count rate of the detector, also referred to as drift. Thus the maximum detectable radiation flux of a direct converter is restricted by polarization. Especially with a high radiation flux density necessary for CT devices the polarization effect occurs to an increased extent.
One approach to a solution is to largely anticipate the polarization of the semiconducting direct-conversion material by irradiating the detector with additional x-ray radiation, in that this additional irradiation is carried out directly before a measurement process. This method however is not suitable for operation with patients since the patient would be subjected to an additional dose. Through the additional x-ray irradiation before the measurement process a preloaded state of the detector is set, the semiconducting direct converter material is thus intentionally polarized, so that both calibration actions and also actual measurement processes can be carried out.
In a further approach to a solution the semiconducting direct-converter material is irradiated directly or indirectly with visible radiation, infrared radiation or UV radiation. This irradiation results in a similar conditioning of the detector to its irradiation with x-ray radiation, wherein the IR radiation is easy to handle and is harmless for the patient.
The direct converter can be irradiated directly by the planar cathode for example. Since the direct irradiation path to the direct converter is restricted by the anti-scatter grid and, for an even irradiation, only a narrow gap is present between the lower side of the anti-scatter grid and the upper side of the semiconductor, as described in the subsequently-published patent application with the file reference DE 10 2012 213 409.3 (the entire contents of which are hereby incorporated herein by reference), there can be provision for an indirect irradiation. A reflection layer is disposed on the anti-scatter grid/collimator for this, which reflects an additional radiation evenly onto the direct converter and is illuminated by a radiation source disposed at the side of the direct converter. The relatively high currents which are needed for illumination mean that the radiation source can heat up strongly, which leads, because of its immediate vicinity to the direct converter or to other electronic components of the x-ray detector, to said components heating up and thus to an undesired change in their sensitivity. In addition a heating up of the radiation source leads to the emitted light power changing.